In general, overlay systems may be provided in cellular or fixed networks for enabling network operators to offer specific services based on and built upon applications, services and protocols not supported in their networks. The intention is that such services will be developed by the network operators and other third party suppliers using the mechanisms provided by external networks, such as the Internet or other external network systems or subsystems.
Recently, an IP Multimedia core network Subsystem (IMS) has been developed which comprises all core network elements for provision of multimedia services. This includes a collection of signalling and bearer related network elements as defined e.g. in the Third Generation Partnership Project (3GPP) specification TS 23.002. IP Multimedia services are based on an IETF (Internet Engineering Task Force) defined session control capability which, along with multimedia bearers, utilizes the PS (Packet Switched) domain of the cellular network. In order to achieve access independence and to maintain a smooth operation with wireless terminals across the Internet, the IMS attempts to be conformant to IETF “Internet Standards”. Therefore, the interfaces specified conform as far as possible to these standards for the case where an IETF protocol has been selected, e.g. SIP (Session Initiation Protocol). The IMS enables the convergence of, and access to, voice, video, messaging, data and to web-based technologies for the wireless user, and combines the growth of the Internet with the growth in mobile communications.
As already mentioned, the IMS utilizes the PS domain to transport multimedia signalling and bearer traffic. The PS domain maintains the service while the mobile terminal moves and hides these moves from the IMS. The IMS is independent of the CS (Circuit Switch) domain although some network elements may be common with the CS domain. This means that it is not necessary to deploy a CS domain in order to support an IMS based network. A User Equipment (UE) (or mobile station (MS)) consists of a mobile equipment (ME with a valid USIM (Universal Mobile Telecommunications System Subscriber Identity Module) (or SIM (Subscriber Identity Module)) attached. In the following, the abbreviation “UE” refers both to MS and user equipment, even those used for emergency calls, which do not have a valid SIM or USIM.
A UE accessing an IMS service requires an IP address which can be logically part of the visited network GPRS IP addressing domain. There are various identities which may be associated with a user of IP multimedia services. Every IMS subscriber has one or more public user identities. The public user identity is used by any user for requesting communications to other users. For example, this might be included on a business card. Both telecom numbering and Internet naming schemes can be used to address users depending on the public user identities allocated to the users. The public user identity may take the form of a SIP URL (Uniform Resource Locator) as defined in RFC 2543 and RFC 2396, or an E.164 number. At least one public user identity may be used to identify the user's information within a subscriber database, e.g. the Home Subscriber Server (HSS), e.g. during mobile terminated set-up.
A location service (LCS) feature in UMTS and GSM (Global System for Mobile Communications) provides a mechanism to support mobile location services for operators, subscribers and third party service providers. Location services may be considered as a network provided enabling technology consisting of standardized service capabilities which enable the provision of location applications. The applications may be service provider specific. In general, LCS is a service concept in a system standardization. LCS specifies all necessary network elements and entities, their functionalities, interfaces as well as communication messages to implement the positioning functionality in a cellular network. Positioning is a functionality which detects a geographical location of e.g. a mobile terminal. Principles and/or algorithms on which the estimation of the geographical location is based may be e.g. AOA, TOA or TDOA. For example, the Global Positioning System (GPS) is based on TOA while OTDOA and E-TD (on GSM) are based on TDOA.
The positioning of the UE is a service provided by the access network. In particular, all access networks (e.g. UMTS Terrestrial Radio Access Network (UTRAN), GSM/EDGE (Enhanced Datarate for GSM Evolution) Radio Access Network (GERAN), which facilitate determination of the locations of UEs, shall be able to exchange location information with a core network as defined in the 3GPP specification TS23.271, when connected to a core network.
By making use of the radio signals, the capability to determine the (geographic) location of the UE can be provided. The location information may be requested by and reported to a client (application) associated UE or by a client within or attached to the core network. The location information may also be utilized internally in a system, for example, for location assisted handover or to support other features, such as home location billing. The position information may be reported in standard, e.g. geographical coordinates, together with the time-of-day and the estimated errors (uncertainty) of the location of the UE according to the 3GPP specification TS 23.032. There are many different possible uses for the location information. As already mentioned, the positioning feature may be used internally by the GSM/UMTS network (or attached networks), by value-added network services, by the UE itself or through the network and by the third parties' services. The positioning feature may also be used by an emergency service. Although LCS has been defined for the CS and PS domains of cellular networks, it has not yet been defined for the IMS domain. Currently, the LCS service does not support IMS LCS clients to be addressed by IMS identities.